Ca2+ and Mg2+‐Dependent Complex Formation of Tropomyosin with Phosphotroponin (P1TI2C) or Dephosphotroponin (TI2C)

Abstract

The reduced viscosity of [rabbit muscle] troponin and dephosphotroponin is independent of the protein concentration in both states, either metal-free or with troponin C saturated with Ca2+ or Mg2+; that of tropomyosin increases linearly as a function of the protein concentration, indicating aggregation. Addition of troponin to tropomyosin increases the reduced viscosity over the expected value being maximal at a 1:1 molar ratio of both proteins. The reduced viscosity of a 1:1 molar mixture of phosphotroponin-Mg4 or dephosphotroponin-Mg3 increases in 2 phases as function of the total protein concentration, indicating the formation of 2 kinds of troponin-tropomyosin complexes. In the 1st phase, troponin and tropomyosin form a non-aggregating 1:1 complex, which is characterized by a value of 0.45 dl/g for the intrinsic viscosity and a sedimentation coefficient of 3.6 S. Employing these 2 values a MW of 150,000 can be calculated, which is in the range of the sum of MW for troponin and tropomyosin (156,000). In the 2nd phase the troponin-tropomyosin complex aggregates further, a process described by: n (troponin-tropomyosin) .fwdarw. (troponin-tropomyosin)n. This further aggregation occurs upon saturation of the Ca2+-specific sites in troponin C. A model is discussed which explains the shortening of 1.5 nm per tropomyosin molecule upon the shift of tropomyosin from the periphery into the groove of the actin filament by tropomyosin aggregation.